Legacy Network classes (please use current Unity Networking). WWW and UnityWebRequest are supported (applies to all scripting backends).

GameObject.SendMessage partially works, but function which accepts the message must match the message sent, because the argument conversion doesn’t work (applies only to .NET scripting backend).

You cannot automatically access C# classes from JS scripts (applies only to .NET scripting backend). To do this, go to the Player settings (menu: Edit > Project Settings, then select the Player category), open the Publishing Settings panel, navigate to the Compilation section and set the Compilation Overrides to .NET Core Partially.